Various methods have been used in the manufacture of photographic products to obtain uniform dispersions of photographically useful hydrophobic compounds, such as color forming couplers, UV-absorbing compounds, dyes and the like.
One method for dispersing a hydrophobic compound in a hydrophilic colloidal coating composition, as described in U.S. Pat. Nos. 2,304,940; 2,332,027 and 2,801,171, comprises dissolving the hydrophilic compound in oil or in a high boiling organic solvent, preferably in the presence of a low boiling auxiliary organic solvent and dispersing the resulting oily solution in a hydrophilic colloidal aqueous solution, preferably a gelatin aqueous solution, by high energy homogenizing. This method, however, requires high energy usage to obtain the required dispersion and particle sizes, which may cause an undesired degradation of the compound. Furthermore, such dispersion technique requires long times and high costs.
Research Disclosure No. 15930 of July 1977 describes a process for dispersing a hydrophobic compound in a hydrophilic coating composition by loading the hydrophobic compound onto the polymeric particles of a loading polymeric latex (for example a polymer obtained by copolymerizing an acrylic acid ester and an acrylamide with at least 2% by weight of an ethylenic monomer containing a sulfonic or sulfonate group). The resulting loaded polymer particles are dispersed in the hydrophilic colloidal coating composition without the need of high energy homogenizations. According to this process, the hydrophobic compound to be loaded is dissolved in a water-miscible organic solvent and an aqueous latex consisting of water, as a continuous phase, and of loading polymer particles, as a dispersed phase, is then blended in the water-miscible organic solvent containing the hydrophobic compound. The use of a loading polymer latex represents a substantial improvement in the art of dispersing hydrophobic compounds in the hydrophilic colloidal layers of photographic materials; the particle sizes of dispersoids containing the loaded polymer particles are lower than those obtained with oil dispersions and the use of high energy homogenization can be avoided. However, the resulting loaded latex dispersions, in general, are not stable if stored for long time periods and the hydrophobic compounds tend to crystallize and after short periods precipitate from the loaded latex. European patent No. 14,921 describes the use of polyurethane latexes to load hydrophobic compounds and form loaded latexes stable for extended times. However, only particular classes of polyurethane latexes proved to be suitable for loading and there were still unsolved problems of compatibility with the hydrophilic colloids.